Fuel cells are electrochemical cells in which the free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Organic fuel cells are a useful alternative to hydrogen fuel cells in many applications, overcoming the difficulties of storing and handling hydrogen gas. In an organic fuel cell, an organic fuel, such as methanol or formic acid (FA), is oxidized to carbon dioxide at an anode, while air or oxygen is simultaneously reduced to water at a cathode. Organic/air fuel cells have the advantage of operating with a liquid organic fuel. Although methanol and other alcohols are typical fuels of choice for direct fuel cells, U.S. Patent Application Publication Nos. 2003/0198852 and 2004/0115518 (both of which are incorporated by reference) disclose formic acid fuel cells with high power densities and current output. Exemplary power densities of 150 mW/cm2 and higher were achieved at low operating temperatures and provided for compact fuel cells. Additional disclosures concerning formic acid fuel cells include WO 2005/081706 and WO 2008/080227 (both of which are incorporated by reference).
Electrocatalysts are catalysts that accelerate electrochemical reactions, without being consumed in the process. Electrocatalysts are useful as catalysts for organic fuel cells. In particular, electrocatalysts, including noble metals with or without admetals, are useful as catalysts for organic fuel cells. An admetal is a metal that modifies the properties of the noble metal catalyst and, therefore, also changes the catalyst properties. The admetal can have a function independent of the noble metal catalyst, can modify the surface of the noble metal catalyst, or can modify the electronic structure of the noble metal catalyst.
The oxidation of formic acid on noble metals, such as platinum or palladium, has been studied extensively in the last decades due to its significance in electrocatalysis of fuel-cell reactions. A generally accepted dual-path mechanism for formic acid oxidation at platinum, proposed by Capon and Parsons, has been supported by in situ infrared (IR) spectroscopy and differential electrochemical mass spectroscopy (DEMS). According to this mechanism the oxidation of formic acid molecules to CO2 is assumed to occur via a reactive intermediate described as *COOHHCOOH→*COOH+H++e−  (1)*COOH→CO2+H++e−  (2)where * indicates the number of platinum sites bonded to the carbon atom of the organic species. The sum of the foregoing reactions,HCOOH→CO2+2H++2e−  (3)represents the so-called direct oxidation pathway, which takes place at the anode. Coupled with the cathode reaction½O2+2H++2e−→H2O  (4)the overall reaction of a direct formic acid fuel cell is given byHCOOH+½O2→CO2+H2O  (5)
In a parallel reaction at the anode, a surface-blocking residue (“poison”) is formed*COOH+HCOOH→***COH+CO2+H2O  (6)In the hydrogen region the formation of the poison is assumed to be especially fast via adsorbed hydrogenHCOOH→*H+*COOH  (7)*COOH+2*H→***COH+H2O  (8)
In a respective way another poison, carbon monoxide, is formed as demonstrated via in situ IR spectroscopy.HCOOH→*COads+H2O→CO2+2H++2e−  (9)
Below the potential of surface oxide formation, poisons inhibit the direct oxidation path. Hence, the oxidation of HCOOH at pure platinum, generally regarded as a good catalyst, proceeds only at a relatively low rate.
Xia et al. (1993) J Electrochem Soc 140(9):2559-65 reported a pronounced catalytic effect of underpotential deposited (“UPD”) lead adatoms on the oxidation of formic acid on a platinum electrode in HClO4. In this report, the electrolyte solution was a 2.11×10−3 M Pb2++0.1 M HClO4 solution.
More recently, Beltowska-Brzezinska et al. (2014) J Power Sources 251:30-7 reported poisoning of a platinum electrode surface by a CO-like species was prevented by suppression of dissociative chemisorption of formic acid due to a fast competitive underpotential deposition of lead adatoms on the Pt surface from an acidic solution containing 10−3 M Pb2+ cations.